An XIntra8 frame (also known as a j-type frame or j-frame) is a type of frame similar to an IFrame. They are used in Microsoft's mpeg4 variation CODEC WMV2/WMV8 and the same coding method (with a different header) is used in WMV3 Complex Profile under the name "X8". XIntra8 frames are a type of intra frame (spatial prediction). This is why they are said to be similar to i-frames, because both b-frames and p-frames use inter-frame prediction (temporal prediction). XIntra8 frames operate on 8x8 pixel blocks. XIntra8 frame types are missing in WMV9. XIntra8 frames reportedly operate in 10 different modes (need more info on this).
MPlayer on x86 can play files containing these frames using a binary codec. ffmpeg can also play files containing Xintra8 frames, but, those frames will be skipped. Xintra8 have not been reverse engineered, so there is no documentation available. If a stream contains Xintra8 frames the Xintra8 bit will be set in the streams extradata.
Note: j-frame is the notiation used by ffmpeg developers. XMV SDK calls it XIntra8 frame and in WMV3 it's called Intra_X8.
The main differences between XIntra8-frames and i-frames:
- They are not derived from MPEG-4 standard
- Picture is coded as blocks, not as macroblocks
- Each frame may use its own Huffman codes
Reference  (in german).